Nitride semiconductor devices made from Group III-nitride semiconductor materials have substantial promise as electro-optical devices such as lasers and light-emitting diodes. The general formula of these materials is AlxGa1-x-yInyN, where Al is aluminum, Ga is gallium, In is indium, N is nitrogen, and x and y are compositional ratios. For instance, these materials have been reported as being used to fabricate lasers that generate light in the blue through ultra-violet part of the spectrum. However, a major practical problem with conventionally-structured lasers fabricated using nitride semiconductor materials is that the far-field pattern of the light generated by such lasers exhibits more than one peak. The far-field pattern is the Fourier transform of the near-field pattern of the light. See, for example, D. Hofstetter et al., Excitation of a Higher Order Transverse Mode in an Optically-Pumped In0.15Ga0.85N/In0.05Ga0.95N Multi Quantum Well Laser Structure, 70 APPL. PHYS. LETT., 1650 (1997). A laser that generates light having a far-field pattern that exhibits multiple peaks has substantially fewer practical applications than a laser that generates light having a far-field pattern that exhibits a single peak.
The far-field pattern of the light generated by such a conventional laser exhibits multiple peaks, rather than the desired single peak, because the optical confinement structure formed by an optical waveguide layer and an underlying cladding layer in the laser provides insufficient optical confinement. The optical confinement structure allows light to leak from the optical waveguide layer into the contact layer underlying the cladding layer. The contact layer then acts as a parasitic optical waveguide that causes spurious laser oscillation in a high-order mode. The contact layer is used to inject current into the active layer of the laser.
Attempts to achieve sufficient optical confinement have included using a cladding layer having an increased thickness, and increasing the refractive index difference between the semiconductor materials of the optical waveguide layer and the cladding layer. However, when implemented conventionally, these measures increase the incidence of cracks in the laser structure. This significantly reduces the production yield.
FIG. 1 is a schematic side view of the structure of the conventional nitride semiconductor layer structure 10 from which conventional nitride semiconductor lasers can be made. In all of the Figures in this disclosure that depict semiconductor layer structures and semiconductor lasers, the thicknesses of the layers are greatly exaggerated relative to their widths, and the thicknesses of the thinner layers are exaggerated more than those of the thicker layers, to enable the layers to be shown clearly.
The nitride semiconductor layer structure 10 is composed of the buffer layer 12 of low-temperature-deposited GaN, the n-contact layer 13 of n-type GaN, the cladding layer 14 of n-type AlGaN, the optical waveguide layer 15 of n-type GaN, the active layer 16 of GaInN, the electron blocking layer 17 of p-type AlGaN, the optical waveguide layer 18 of p-type GaN, the cladding layer 19 of p-type AlGaN and the p-contact layer 20 of p-type GaN. The layers 12 through 20 are successively formed on the substrate 11. The material of the substrate is sapphire, SiC, spinel, MgO, GaAs, silicon, or some other suitable material.
The buffer layer 12 is a 35 nm-thick layer of GaN deposited at a temperature of 550xc2x0 C., i.e., at a temperature below that at which single-crystal growth occurs. The n-contact layer 13 is a 4 xcexcm-thick layer of GaN doped n-type with about 1xc3x971018 cmxe2x88x923 of Si. The cladding layer 14 is a 600 nm-thick layer of AlGaN having an AlN molar fraction of 0.06 and doped n-type with about 1xc3x971018 cmxe2x88x923 of Si. The optical waveguide layer 15 is a 100 nm-thick layer of GaN doped n-type with about 1xc3x971018 cmxe2x88x923 of Si. The active layer 16 is composed of five pairs of sub-layers. Each pair of sub-layers is composed of a 3 nm-thick sub-layer of GaInN having an InN molar fraction of 0.1 and a 6 nm-thick sub-layer of GaInN having an InN molar fraction of 0.03. The electron blocking layer 17 is a 15 nm-thick layer of AlGaN having an AlN molar fraction of 0.15 and doped p-type with about 5xc3x971019 cmxe2x88x923 of Mg. The optical waveguide layer 18 is a 100 nm-thick layer of GaN doped p-type with about 5xc3x971019 cmxe2x88x923 of Mg. The cladding layer 19 is a 500 nm-thick layer of AlGaN having an AlN molar fraction of 0.06 and doped p-type with about 5xc3x971019 cmxe2x88x923 of Mg. The p-contact layer 20 is a 100 nm-thick layer of GaN doped p-type with about 1xc3x971020 cmxe2x88x923 of Mg.
An improved nitride semiconductor layer structure from which can be made nitride semiconductor lasers that generate light having a far-field pattern that exhibits a single peak is described in Japanese Patent Application no. H10-313993 (the prior application). The prior application is assigned to the assignee of this application and was filed on Oct. 16, 1998. An English language version of the prior application is published as International Application no. WO 00/24097. The prior application is incorporated into this disclosure by reference.
The nitride semiconductor layer structure disclosed in the prior application lacks the electron blocking layer 17 of the layer structure shown in FIG. 1, but includes an additional buffer layer located between the n-type GaN contact layer 13 and the n-type AlGaN cladding layer 14. The additional buffer layer is a layer of low-temperature-deposited semiconductor material that includes AlN. The additional buffer layer enables the n-type AlGaN cladding layer 14 grown on it to have an increased thickness and a lower incidence of cracks. In the nitride semiconductor layer structure disclosed in the prior application, either or both of the thickness of the cladding layer and the AlN molar fraction of the cladding layer can be adjusted to make the light emitted by a laser fabricated using the layer structure to have a far-field pattern, i.e., the intensity distribution in the far field, that exhibits a single peak.
Although the layer structure disclosed in the prior application can be used to fabricate lasers that generate light whose far-field pattern exhibits a single peak and that have a greater efficiency than conventional nitride semiconductor lasers, the need exists for high-quality lasers having a simpler structure.
Thus, what is needed is a nitride semiconductor layer structure that has a simple structure and from which a nitride semiconductor laser can be fabricated that generates light whose far-field pattern exhibits a single peak, and that has a low threshold current and a low power consumption.
What is also needed is a nitride semiconductor layer structure capable of providing increased optical confinement, that has a reduced manufacturing cost and that provides improved performance in opto-electric devices, other waveguide structures and other semiconductor devices.
The invention provides a nitride semiconductor layer structure that comprises a buffer layer and a composite layer on the buffer layer. The buffer layer is a layer of a low-temperature-deposited nitride semiconductor material that includes AlN. The composite layer is a layer of a single-crystal nitride semiconductor material that includes AlN. The composite layer includes a first sub-layer adjacent the buffer layer and a second sub-layer over the first sub-layer. The single-crystal nitride semiconductor material of the composite layer has a first AlN molar fraction in the first sub-layer and has a second AlN molar fraction in the second sub-layer. The second AlN molar fraction is greater than the first AlN molar fraction.
The invention additionally provides a nitride semiconductor laser that comprises a portion of the above-described nitride semiconductor layer structure, and that additionally comprises an optical waveguide layer over the composite layer and an active layer over the optical waveguide layer.
In the simple nitride semiconductor layer structure provided by the invention, the thick, composite AlGaN layer may be doped p-type or n-type. The composite AlGaN layer has an AlN molar fraction that changes through the thickness of the layer to define two sub-layers in the composite AlGaN layer. The different AlN fractions of the sub-layers optimize the characteristics of the sub-layers to enable the sub-layers to function as a cladding layer and as a contact layer. Accordingly, the sub-layers will be called a cladding sub-layer and a contact sub-layer, respectively. The cladding sub-layer and an optical waveguide layer located over the composite AlGaN layer form an optical confinement structure that provides adequate optical confinement. The composite AlGaN layer has a relatively high AlN molar fraction in the cladding sub-layer. The composite AlGaN layer has a relatively low AlN molar fraction in the contact sub-layer to give the contact sub-layer the high conductivity required for it to effect lateral current injection without an excessive forward voltage drop. Providing the functions of the contact layer and the cladding layer in a composite layer of a material in which only the AlN molar fraction differs provides a substantial reduction in the incidence of cracking in the nitride semiconductor layer structure.
The nitride semiconductor lasers according to the invention fabricated from the above-described nitride semiconductor layer structure have a high production yield because the composite AlGaN layer has a low incidence of cracks despite its substantial thickness. Such lasers generate light whose far-field pattern exhibits a single peak because of the ample optical confinement resulting from the relatively large AlN molar fraction and thickness of the cladding sub-layer of the composite AlGaN layer. Such lasers also have a low forward voltage drop because of the high conductivity and substantial thickness of the n-contact sub-layer of the composite AlGaN layer.
Because lasers according to the invention generate light whose far-field pattern exhibits a single peak, such lasers are ideal for use in optical information recording devices and other applications that require a single-peak emission profile. Also, since such lasers have a greatly-reduced forward voltage drop, a lower power consumption and dissipate less heat, they have a significantly increased service life.